Why does my dog eat dandelions?

I have a shetland sheepdog (http://flickr.com/photos/elliottcable/tags/josh/) who just LOVES to eat dandelions. He'll always snap them up when he thinks i'm not looking on walks, and he'll usually try to get 20 or 30 in during a single 30 minute walk.

Lately he's even taken to ignoring the heads and just eating the stems. I'm starting to think there's something addictive in that glue sap stuff in the stems - does anybody know anything about this?